Sputtering, alternatively called physical vapor deposition, is widely used to deposit layers of target material on a wafer or other substrate. Plasma sputtering is most often used in the fabrication of electronic integrated circuits. A plasma, typically formed of an inactive working gas, is formed adjacent to the target and plasma ions are electrically attracted to the target at sufficient energy to dislodge or sputter target atoms from the target which then deposit on the wafer in opposition to the target. A magnet assembly, most often called a magnetron, is usually placed in back of the target to create a magnetic field adjacent and parallel to the front face of the target. The magnetic field traps electrons and thereby increases the density of the plasma and hence the sputtering rate.
In the recent past, sputtering in commercial production has largely involved sputtering of highly conductive metals, such as aluminum, copper, and titanium, to form electrical interconnects and refractory barrier layers. In these applications, a DC electrical potential has been conventionally applied to the target and the magnetron typically has nested magnetic poles of an outer pole of one magnetic polarity surrounding an inner pole of the opposed magnetic polarity and separated from it by an annular gap. The magnetic field between the two poles forms a magnetic tunnel which supports and guides plasma current in a closed loop, thereby forming a dense plasma. Advantageously, the magnetron is made relatively small to increase the local plasma density, but the magnetron then needs to be rotated about the back of the target to produce more uniform sputtering.
In many application involving very high plasma densities and ionized sputtered atoms, the small magnetron is positioned near the outer periphery of the target and rotated about the target center so that the sputtered ions diffuse toward the central axis to produce a more uniform deposition on the wafer. As a result, the target center is not being significantly sputtered and instead some of the sputtered atoms redeposit on the central area of the target. In other configurations, the target center is sputtered and the outer regions of the target are subject to redeposition. Such redeposited material is not stable and is likely to produce deleterious particles which falls on the wafer and create defects, thereby reducing the yield of operable integrated circuit dies.
To minimize problems from redeposition, a magnetron can be moved from one radius on the target at which a production wafer is being coated to another radius at which redeposited portions of the target are being cleaned with no production wafer being present. See, for example, U.S. Pat. No. 6,228,236 to Rosenstein et al., U.S. Pat. No. 7,736,473 to Miller et al., and U.S. Pat. No. 7,767,064 to Pavloff et al. In U.S. Pat. No. 8,021,527, Miller et al. describe a more general mechanism for varying a magnetron radius while it is azimuthally rotating about the target center. These patents are incorporated herein by reference for alternative mechanisms for radially moving all or part of a magnetron.
Although the technology dates back many decades, RF (radio frequency) sputtering has been recently promoted for sputtering into very narrow holes, such as vias, to achieve reasonable step coverage so that the material is deposited deep within and on the sides of the hole. See US patent application publication 2010/0252417 to Allen et al. RF sputtering benefits from a significantly different magnetron than that used in DC sputtering. In one embodiment, the magnetron is formed by a relatively large magnet assemblies of two opposed and spaced poles arc-shaped in large segments about the rotation axis and having open ends rather than a closed shape. The open-ended shape may be described as an open loop versus the closed loop of conventional DC magnetron. The present inventors believe that such an open-loop magnetron leaks plasma out of the ends of the magnetron to thereby increase the depth and volume of the plasma so that neutral metal atoms sputtered from the target are more likely to be ionized as they traverse the plasma in their trajectory to the wafer. Sputtered metal ions are particularly beneficial for bottom coverage in vias of high aspect ratio when the wafer is negatively biased.
Wang et al. in US patent application publication 2011/01311735 suggest a magnetron having a spiral shape about the target center with a varying gap between the poles of the magnetron.
Allen et al. in the previously cited publication have recognized the need to clean the inner portion of the target and have described an embodiment of a magnetron formed in a partial circle about the rotation axis during sputter deposition processing which can be pivoted inwardly to cross most of the diameter of the target during target cleaning.